The primary function of the pulmonary circulation is gas exchange. Therefore, it would be particularly advantageous to the organism if this function was achieved with minimal energy expenditure at maximal efficiency. The coupling between the heart and the low pressure pulmonary circulatory system appears to be designed to meet this objective. The capability of the pulmonary vasculature to regulate its interaction with the heart has been recognized. Several hypotheses have been proposed but have not been tested by experiment. The objective of this project is to test these hypotheses and develop a unifying hypothesis that will describe the effect of the pulmonary vascular response to acute circulatory disturbances on the coupling between the pulmonary circulation and the heart. The specific aims of this project are to measure the pulmonary vascular response to various circulatory disturbances in the anesthetized cat, to determine if active mechanisms are involved and to evaluate its effect on coupling between the pulmonary circulation and the heart. In some experiments a programmable pump that can accurately reproduce pulmonary arterial pressure will be used to simulate right ventricular function. The pulmonary vascular response will be assessed in terms of pulmonary vascular resistance, pulmonary arterial compliance, characteristic impedance and wave transmission time. The circulatory disturbances that will be used include alteration of right ventricular performance, raised left atrial pressure, pulmonary vascular occlusion, and alteration of the phase relation between pulse waves in the lesser and greater circulations. This project will result in a better understanding of the behavior of the pulmonary vasculature during acute circulatory disturbance and may lead to improved therapeutic strategies for patient management in the critical care unit.